Solid-state diffusion-based processing kinetics for uniaxial self-propagating high-temperature synthesis of MoSi2

Abstract

An analysis is presented for self-propagating, high-temperature synthesis (SHS) of MoSi2 from a mixture of silicon and molybdenum powders compacted into a semi-infinite, uniaxial bar and ignited at one end. The kinetics of reacting molybdenum grains are controlled by solid-state diffusion through the interposing product shell of MoSi2 that surrounds each shrinking molybdenum grain. The previously determined temperature-dependent microkinetics of this solid particle reaction are coupled with one-dimensional (1-D) heat transfer and storage to describe the time-dependent macrokinetics of the synthesis reaction sequencing through a perfectly insulated bar. The resulting equations are solved numerically to provide computed results of temperature and conversion as a function of time and distance. Preignition time, propagation velocity, and thickness of the reaction front are also determined. Results depend primarily on the initial temperature at the end of the bar, which affects preignition time, and the molybdenum grain radius. The perfectly insulated model was relaxed by limiting the maximum temperature to arbitrary values corresponding with lateral heat dissipation, and these results compare favorably with experimentally measured propagation velocities and maximum temperatures during SHS of MoSi2. The model presented, with MoSi2 as the prototype, is expected to be applicable to the SHS of many other refractory materials.